Aqueous ink compositions

The present application is a § 371 National Phase application based on PCT/GB2021/052578 filed Oct. 6, 2021, which claims the benefit of U.S. Provisional Application No. 63/091,377 filed Oct. 14, 2020, the subject matter of each of which is incorporated by reference in their entirety.

The present invention relates to aqueous ink compositions and methods of preparing them, as well as methods of printing such compositions and printed articles produced thereby. The present invention particularly relates to inkjet printing and inkjet printing ink. The present invention particularly relates to resoluble, yet crosslinkable aqueous ink compositions, especially inkjet but also other types of inks such as for example aqueous flexographic and gravure printing inks. The inks of the present invention represent a significant advance for ink technology, especially inkjet ink technology, and especially for inks where the amount of humectant co-solvent needs to be restricted to enable faster single pass printing. The inventors have shown that inks prepared according to the present invention show excellent stability, showing very little change in viscosity or pH when stored at 50° C., for 2 weeks. This is an especially advantageous feature for inkjet printing where any changes in viscosity can be detrimental to the printing (jetting) quality. A further benefit of this stable ink technology is that when the inks partially or fully dry at temperatures used in printing (typically up to 40° C.) the inks can be re-dissolved quite readily, reducing the risk of irreversibly blocking printheads.

The use of self-crosslinking styrene-acrylic dispersions in conventional, analogue, printing processes is well known. A good overview of the keto-amine chemistry of this type of polymer dispersion is provided by N. Kessel et. al. (J. Coat. Technol. Res. (2008), (5), 285). The polymer contains ketone, or aldehyde, groups as part of its molecular structure which can then react, upon drying, with multifunctional primary (or secondary) amines to affect the crosslinking reaction. A typical multifunctional amine used in self-crosslinking styrene-acrylic emulsions is adipic dihydrazide, although any other multifunctional amine that can react with either carbonyl or aldehyde groups may also be used.

Polyurethane dispersions (PUDs) are seemingly the predominant resin chemistry used in the preparation of pigmented aqueous inkjet printing inks. There are a number of instances in the prior art of the use of amino resins, such as melamine-formaldehydes, to crosslink aqueous inkjet printing inks containing PUDs, especially in the printing of textiles. WO2009/137753 describes how PUDs can be crosslinked with Cymel 303, a melamine-formaldehyde crosslinker, at a temperature of 160° C. A number of other patents describe the use of optional crosslinkers in combination with PUDs to enable improved resistance properties, including; WO2019074683, US20190106587, U.S. Pat. No. 10,513,622, JP2002294112. U.S. Pat. No. 9,249,324, which discloses polyurethane pigment dispersants, usefully lays out the crosslinking possibilities for polyurethanes having any of carboxylic acid, hydroxyl or amine pendant functional groups. Crosslinkers include carbodiimides, epoxies, isocyanates, amino resins (e.g. melamine-formaldehyde), aziridines. However, these references do not disclose the use of self-crosslinking styrene-acrylic dispersions (or solutions), nor their combination with hydroxyl-functional PUDs to deliver a resoluble, yet crosslinkable inkjet technology.

US20170029639 and US20170218565 describe the preparation of encapsulated resin particles, where the capsules may comprise compositions which are thermally curable. Again, various conventional crosslinking chemistries are disclosed including; ketone-hydrazine, epoxy-amine, melamine-formaldehyde, with a preference for the use of blocked isocyanates. Again, no reference to the preferential use of hydroxyl-functional PUDs to promote the resolubility of the inks is disclosed.

WO2017021278 describes composite resin particles comprising low molecular weight conventional crosslinkers, such as epoxies, oxetanes, aziridines, amino resins (such as melamine-formaldehydes) and blocked isocyanates, where the molecular weight of the crosslinker is preferably less than 2000 Daltons.

WO2020102788 discloses aqueous inkjet printing inks comprising carboxylic acid functional acrylic resins which may be crosslinked with well-known, conventional, crosslinkers such as polycarbodiimides, polyoxazolines, aziridines, melamine-formaldehydes, ammonium zirconium carbonate. Again, no mention of the use of self-crosslinking styrene-acrylic dispersions was made.

It should be understood that the present invention may optionally use crosslinking agents as disclosed in the prior art. Thus, the aqueous inkjet compositions of the present invention may comprise hydroxyl-functional PUDs and self-crosslinking styrene-acrylic dispersions (the terms ‘emulsion’ or ‘latex’ are often used in place of dispersion), along with any crosslinker, such as for example polycarbodiimide, polyoxazoline, polyaziridine, a metal complex crosslinker (including titanate and zirconate organometallics, such as ammonium zirconium carbonate) amino resin (e.g. melamine-formaldehyde), blocked isocyanate, epoxy crosslinker, etc.

Aqueous inkjet compositions comprising (meth)acrylic copolymer dispersions are also well known. U.S. Pat. Nos. 7,338,988, 7,638,561, 7,354,476 describe aqueous inkjet compositions comprising acrylic dispersions having a Tin the range −40 to 150° C. U.S. Pat. No. 7,638,561 and JP2015093954 describe the use of core-shell acrylic dispersions.

Recently, the use of acrylated polyurethane dispersions (‘Ac-PUDs’) in the preparation of UV-, and EB-curable aqueous inkjet printing compositions has been disclosed quite widely, for example in U.S. Pat. No. 10,076,909, EP3390545, WO2017174981 and WO2018138525. The crosslinking of these Ac-PUDs is induced by the use of a suitable photoinitiator in the case of UV-curing. In principle, such compositions will have excellent stability since crosslinking can only be affected in the presence of intense UV light or EB radiation once the ink composition has been partially or fully dried after printing. However, the use of photoinitiators can pose a risk of unwanted migration of low molecular weight compounds from a cured ink, which would be undesirable in many applications, especially the printing of food packaging. Furthermore, Ac-PUDs can be prone to hydrolysis of the acrylate groups, resulting in the liberation of acrylic acid causing potential pH instability issues, again undesirable for inkjet printing.

A number of references describe the use of self-crosslinking styrene-acrylic dispersions in the preparation of aqueous inkjet ink compositions, but none of the identified prior art discloses the beneficial use of hydroxyl-functional PUDs in combination with such dispersions. WO2001036547 describes compositions comprising self-crosslinkable polymer (or oligomer) emulsions containing carbonyl groups as part of the polymeric structure, which can crosslink with adipic dihydrazide, for instance. The compositions further comprise what is referred to as a “resolubilizing polymer”, such as a water-soluble acrylic polymer. The inventors have found that such compositions, although delivering excellent print resistance, have very poor resolubility when tested in the manner detailed by way of the examples. Certainly, the innate resolubility achieved with hydroxyl-functional PUDs, as disclosed herein, is a highly beneficial aspect of the invention.

JP2006045334, JP2004149600, JP2009149774 and JP2012241135 also describe aqueous inkjet ink compositions comprising self-crosslinking styrene-acrylic dispersions, but again without reference to the beneficial use in combination with hydroxyl-functional PUDs to promote the ink resolubility.

CN110982344 discloses white aqueous inkjet ink compositions comprising self-crosslinking acrylic emulsions, along with a PUD and also a further crosslinker, such as a blocked isocyanate. This prior art fails to disclose or allude to the benefit arising from the use of hydroxyl-functional PUDs.

WO2018138720 describes crosslinkable inkjet compositions which may comprise a silane-functional self-crosslinking PUD. JP2006264228 and US20070109375 disclose inks comprising acetoacetate-functional poly(vinyl alcohol)s which may crosslink with multifunctional amines (including dihydrazides).

The literature (and that which is understood by the inventors to be currently practiced commercially) has not satisfactorily described how to achieve resoluble aqueous inkjet ink compositions comprising self-crosslinkable styrene-acrylic polymer dispersions, which at the same time deliver prints that can cure at temperatures of 100° C. or less, even at room temperature, after the prints have been dried. This is especially the case for inks prepared according to the invention, which are intended for single pass inkjet printing, with concentrations of co-solvents such as propylene glycol of 30% (w/w), or less. The key feature of the invention in achieving this has been to combine self-crosslinkable styrene-acrylic polymer dispersions (or aqueous solution polymers) with hydroxyl-functional PUDs, believed to be the first reported instance of this approach, not just for inkjet applications but more broadly for other ink types. Thus, the invention although directed primarily towards inkjet also finds utility in other printing and coating applications, including flexographic printing.

The resolubility of the compositions can be further enhanced by the optional use of amines having boiling points in the preferred range of 130 to 250° C. and yet more preferably in the range 130 to 200° C. Such amines not only promote the resolubility of the inks, they still allow prints to cure after being dried or heated at temperatures of 100° C., or less. This is not the case with involatile amines such as triethanolamine, which is a commonly used amine in the art.

To summarize, the key advantages of the inventive technology are:

Citation or identification of any document in this application is not an admission that such represents prior art to the present invention.

According to the present invention, there is provided curable aqueous inkjet printing ink compositions, having excellent resolubility, comprising self-crosslinkable styrene-acrylic polymer dispersions and hydroxy-functional PUDs.

The invention resides in the finding that aqueous printing inks having excellent resolubility/redispersibility but with the capacity to cure after printing to produce water resistant prints can be prepared by combining a self-crosslinking styrene-acrylic dispersion or solution polymer (‘SCSA’) with a hydroxyl-functional polyurethane dispersion (‘OH-PUD’). The use of the OH-PUD confers the resolubility/redispersibility of the inks, which is an especially useful feature for aqueous inkjet printing inks. Resolubility/redispersibility, in the context of this invention and inkjet more broadly, can be described as the ability of an ink drying at temperatures up to 40, and even 50° C., for periods up to one hour, to re-dissolve, or redisperse, into itself or a suitable ‘flushing’ solution. Without the OH-PUD, SCSAs have poor resolubility/redispersibility, as will be shown by way of the examples. For inkjet printing, the invention is exceptionally advantageous as the good resolubility helps to prevent the irreversible drying of inks in a printhead. If such irreversible drying of an ink in a printhead occurred it could lead to blocked nozzles and a consequent loss in print quality performance or, even worse, the loss of the printhead itself. So, it can be seen that an ideal solution for an inkjet printing ink would be one that has good resolubility (with advantages of open time) yet at the same time being able to cure after printing and drying to produce water resistant prints, especially at temperatures of 100° C., or less. The invention achieves this without recourse to the use of excessive concentrations of high boiling point co-solvents such as glycerol and propylene glycol. For the purposes of this application, a high boiling solvent has a boiling point of at least 250° C. (under a standard pressure of 100 kPa). Indeed, the inventive inks maintain good resolubility/redispersibility with propylene glycol concentrations as low as 30%, such as 25%, or 15% (w/w) of the ink composition. Thus, a further aspect of the invention is that the concentration of organic co-solvents should preferably be 30% or less, such as less than 30%, and more preferably 25% (w/w) or less of the total ink composition. Where such co-solvents are used, then any combination of solvents having boiling points of greater than 200° C. should preferably be less than 10% (w/w) of the ink composition.

The inventors have also found that the resolubility of the inks can be further enhanced by using amines having boiling points greater than 130° C. However, if amines with boiling points greater than 250° C. are used, such as triethanolamine, although improved ink resolubility is achieved, the cure of the printed inks can be adversely affected. However, where amines with boiling points of less than 250° C. are used, such as N,N′-dimethylethanolamine, N-methylethanolamine, 2-Amino-2-methyl-1-propanol and N-methyldiethanolamine, not only is the resolubility enhanced but the cure of dried prints develops much more rapidly than triethanolamine, for example. The amines can be introduced to the inks either as an additive to the ink, or by using the amines in the neutralisation of the SCSAs and OH-PUDs of the invention. The inventors have found that neutralising an SCSA with such an amine can be effective in improving the resolubility of an ink.

The SCSAs of the invention may use any self-crosslinking chemistry as described previously, including the well-known keto-dihydrazide chemistry; as well as self-condensation of polymers comprising N-methylol functionality; the crosslinking of polymers comprising carboxylic acid or carbonyl groups with metal complex crosslinking agents; and self-condensation of polymers comprising organosilane functionality. The keto-dihydrazide crosslinking chemistry is well established and is used in a number of applications, including waterbased flexographic printing inks. Furthermore, there is a large number of commercial grades which are suitable for the printing of food packaging. The issue with using SCSAs by themselves in the preparation of aqueous inkjet ink compositions is that when they are dried at temperatures up to 40° C., for an hour, they become largely insoluble in either the ink itself or a flushing solution. It is the introduction of the OH-PUD which helps to improve the resolubility/redispersibility of the inks whilst allowing them to cure, after printing and drying, to produce water resistant prints. Contrariwise, OH-PUDs, especially those having preferred hydroxyl values of greater than 50 mgKOH/g, can produce inks having excellent resolubility/redispersibility, but which generally produce dried prints with poor water resistance. It is the novel combination of OH-PUDs and SCSAs that can produce aqueous printing inks, especially aqueous inkjet printing inks, with a desirable combination of resolubility/redispersibility and cure of a dried print.

After printing and drying, the inks of the invention can develop their cure at ambient or elevated temperatures. The inventors have found that at ambient conditions the cure of the inks can develop for a day or more after printing. The cure of the inks can be accelerated by heating the prints at temperatures of 50° C., or greater. Prints of inks prepared according to the invention can achieve excellent water resistance when heated to temperatures of 100° C., or less.

The inkjet printing inks of the invention may be used in any application including, but not limited to; packaging printing (flexible, cartonboard and corrugated); metal decoration printing (including metal can packaging); textiles printing; and décor printing.

As well as monomers comprising the crosslinking function; whether that be aldehyde, ketone, carboxylic acid, methylol or silane group, the SCSAs of the invention may further comprise any of the following, non-limiting monomers in their preparation; styrene, methyl-styrene, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, hexyl acrylate, hexyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate, or any ester of acrylic or methacrylic acid, acrylic acid, methacrylic acid, itaconic acid, maleic anhydride, styrene sulphonic acid and its salts, 2-Acrylamido-2-methylpropane sulfonic acid and its salts, vinyl acetate, any acrylamide or methacrylamide monomer. It should be noted, however, that the invention covers any aqueous self-crosslinking styrene-acrylic polymer. It is the combination with the OH-PUD and optional amine of the invention which is key in delivering resoluble inks, especially for inkjet printing.

The hydroxyl-functional polyurethane dispersion (‘OH-PUD’) may have a hydroxyl value, based on the dry weight of the polymer, of at least mgKOH/g, such as at least 20 mgKOH/g, at least 25 mgKOH/g, or at least 30 mgKOH/g, and preferably 50 mgKOH/g or greater. For example, the OH-PUD may preferably have a hydroxyl value, based on the dry weight of the polymer, of 70 mgKOH/g or greater, such as 90 mgKOH/g or greater, 110 mgKOH/g or greater, 130 mgKOH/g or greater, 150 mgKOH/g or greater, or 170 mgKOH/g or greater. Such PUDs may be prepared by methods well known to those skilled in the art, and the high hydroxyl values of the PUD may be achieved by using an excess of diol, compared to diisocyanate in the polyurethane preparation or by using chain terminating species such as ethanolamine and diethanolamine to end-cap the polyurethane. For the purposes of the current invention it is not important how the OH-PUD is prepared other than that it enables a resoluble/redispersible ink to be prepared. Furthermore, there is no restriction on whether the OH-PUD is anionically or non-ionically stabilized. In the first case, the OH-PUD comprises acid groups as part of its polymeric structure which, after neutralization with a suitable base, confer the anionic stabilization mechanism enabling its dispersion. In the case of non-ionically stabilized PUDs it is usual to incorporate a hydrophilic segment as part of the polyurethane dispersion (such as a poly(ethylene oxide)) to enable its dispersion. Where the PUDs are anionically stabilized it is preferred that the acid value, based on the dry polymer weight, should be at least 10, and more preferably at least 20 mgKOH/g. Dispersion of OH-PUDs may also be promoted by the use of anionic and non-ionic surfactants.

There is no particular restriction on the total solid content contributed by the OH-PUD and SCSA to the final aqueous inkjet printing ink composition. However, it is preferred that the total solid content contributed by both components to the final ink formulation should be in the range 2.5% (w/w) to 30% (w/w), and more preferably in the range 5.0% (w/w) to 20.0% (w/w) of the final ink composition, based on the dry polymer weight.

There is no particular restriction on the ratio of the SCSA to the OH-PUD, based on dry weight. However, it is preferred that this ratio should be in the range 10:1 to 1:10 and more preferably in the range 8:2 to 2:8.

The aqueous inkjet printing inks of the invention preferably have viscosities, at 32° C., of less than 10.0 mPa·s. Unless stated otherwise, the viscosities of the inks were measured using a Brookfield DV-II+ Pro Viscometer equipped with Spindle no. 18, 100 rpm, 32° C.

The pH of the inks should preferably be in the range 5.0 to 10.0 and more preferably in the range 6.0 to 9.5.

The inks of the invention may optionally comprise any additional crosslinker, including but not limited to those comprising the following reactive species; carbodiimides, oxazolines, aziridines, epoxies, amino resins (such as melamine-formaldehydes), metal complexes (including titanate and zirconate organometallics, such as ammonium zirconium carbonate), isocyanates (including blocked isocyanates), epoxies.

A further, optional, feature of the invention is that where any water-soluble organic solvent is used, it should preferably have a boiling point of less than 250° C., and more preferably less than 200° C. Where solvents with boiling points of greater than 250° C. are used their concentration in the ink should preferably be less than 10.0% (w/w) and more preferably less than 5.0% (w/w). The total concentration of water-soluble organic solvents should preferably be less than 35.0% (w/w), and more preferably less than 30.0% (w/w) of the ink composition, such as 25.0% (w/w) or less.

The inks of the invention may be printed via multipass or single pass inkjet printing processes, terms which are well understood in the industry. The inks are particularly advantageous in single pass inkjet printing.

The inks of the invention may be printed onto any suitable substrate, either unprimed or which has previously been pre-coated with a suitable primer composition to enhance the print quality. Such primers typically comprise a multivalent metal salt which helps to ‘fix’ the ink, reducing print quality issues such as drop spread and intercolour bleed. It should be understood that although primers do not form a key part of the invention any suitable primer enhancing the print quality achievable with the inventive inks may be used.

The printing ink compositions may further optionally include an amine having a boiling point preferably greater than 130° C., but less than 250° C., such as less than 220° C., less than 200° C., or less than 180° C.

After printing and drying the inks of the invention, they can cure at room temperature or at elevated temperatures, for example 50° C., or greater.

The inks of the invention are suitable for a wide range of applications, including the printing of packaging, including flexible film packaging, rigid packaging, cartonboard and paper packaging, the printing of décor laminates, textiles printing, graphics printing. When the inks of the invention are used to print packaging materials this may also include food packaging.

A key finding by the inventors is that the resolubility of inks comprising self-crosslinkable styrene-acrylic dispersions can be dramatically improved through the inclusion of a hydroxyl-functional PUD. At the same time, once the inks have been printed, dried and cured they can develop excellent (water) resistance, a feature in which hydroxyl-functional PUDs are deficient. To enhance the resolubility of the inks yet further the inventors have found that the optional addition of amines with boiling points preferably greater than 130° C. can induce a further improvement. Resolubility is a highly desirable feature, especially for inkjet printing applications, and denotes the ability of an ink to redissolve or redisperse into itself, or a suitable flushing solution, after it has partially, or fully dried, at temperatures of 40° C., or less. For the inkjet printing of aqueous inks, printheads are usually maintained at temperatures of 35° C., or less. To demonstrate that the inks of the invention are resoluble, the inventors dried the inks at 40° C. for up to one hour, and then assessed their resolubility/redispersibility into either deionized water or a varnish simulant of the ink. If an ink was to irreversibly dry in the nozzles of an inkjet printhead it would cause blockage of the nozzles with consequent loss of printing quality and potentially catastrophic and expensive loss of the printhead itself. This would be a significant risk for an aqueous inkjet composition comprising solely of a self-crosslinking styrene-acrylic dispersion. The inventors have shown, by way of the previously mentioned resolubility tests, that inks comprising solely of self-crosslinking styrene-acrylic emulsions do not readily redissolve/redisperse and could well cause the unwanted blockage of printheads.

The invention applies to a wide range of self-crosslinking chemistries for the styrene-acrylic polymer component of the invention. Parvate et. al. (J. Disp. Sci. Tech, 40 (2019), Issue 4, p. 519) usefully describe these chemistries, which can include; (i) the self-condensation of N-methylol functionality on the polymer backbone, (ii) the reaction of ketones (or aldehydes) attached to the polymer with hydrazides or amines, (iii) the self-condensation of organofunctional silanes attached to the polymer, (iv) metal complex (salts, chelates) reaction with polymer functional groups such as acetoacetoxy groups or carboxylic acid groups. For the printing of food packaging the carbonyl-amine crosslinking chemistry as in (ii) above has been established for well in excess of ten years. More recently, the stability issues surrounding (iv), for example the crosslinking reaction between carboxylic acid groups on the polymer backbone and a metal complex crosslinking agent, such as a titanate or zirconate organometallic, like ammonium zirconium carbonate, have been successfully resolved (U.S. Pat. No. 7,947,760).

Thus, in essence, the invention discloses an ideal solution for the printing of aqueous inkjet compositions; resoluble inks that, after printing, drying and curing, can produce highly water resistant prints. Furthermore, although the cure/crosslinking of the inks can be more rapidly achieved at elevated temperatures, 50° C. or more, prints produced using inks according to the invention can cure, at ambient temperatures, over a period of one to seven days.

To enhance the resolubility of the inks yet further, the inventors have found that the use of amines with boiling points in excess of 130° C., have a positive impact. Where optional amines are used in the inventive compositions this may be achieved either through additive addition to the ink composition. Alternatively, the polymer dispersions (and solution polymers), including the polyurethane and self-crosslinking styrene-acrylic dispersions/solutions of the invention, can be neutralized with the amines according to the invention. The inventors have also found that amines having boiling points less than 250° C. are even more preferable, since not only do they enhance the resolubility of the inventive inks they also enable the inks to crosslink effectively after drying. When an involatile amine, such as triethanolamine, with a boiling point of around 335° C., is used in compositions according to the invention, although the ink resolubility is enhanced, it can retard the cure of an ink print, especially at temperatures of 100° C., or less. The inventors believe that this is due to such involatile amines persisting in the dried print thereby limiting the curing performance. This will be shown by way of the examples. However, when amines such as N-methyldiethanolamine, boiling point of around 247° C., N-methylethanolamine, boiling point of around 160° C., 2-Amino-2-methyl-1-propanol, with a boiling point around 165° C., and N,N-dimethylethanolamine, boiling point of around 133° C. are used, not only is the resolubility enhanced but the inks more readily cure (crosslink) after drying than is the case with inks comprising triethanolamine (which is an amine still encompassed by the invention).

The addition of the amines according to the foregoing will also help to delay the onset of cure of self-crosslinking styrene-acrylic dispersions (and solutions) relying on the carbonyl-amine and carboxylic acid/carbonyl-metal complex crosslinking reactions, described above. Using an amine that persists in a drying ink will help delay/slow the crosslinking reaction between the reactive groups contained within the backbone of the self-crosslinking styrene-acrylic dispersion and the corresponding crosslinking agent contained in the aqueous phase of the dispersion. However, by using amines according to the invention (with boiling points preferably in the range 130 to 250° C., and even more preferably in the range 130 to 200° C.) allows the amine to evaporate from the ink print during drying to allow the crosslinking reaction to more rapidly occur. However, it should be stressed that the use of amines according to the foregoing is an additional, optional, feature of the invention.

The commercial advantages of this technology are very much related to the technical advantages. Although UV-curable and thermally curable aqueous inkjet compositions are well known, the technology described here opens up the potential for aqueous inkjet to deliver desirable performance in a number of applications, including; packaging, décor, textiles, graphics, etc. By overcoming the conundrum of providing an ink with excellent resolubility which at the same time can cure and develop useful print resistance properties at temperatures of 100° C. or less, even at room temperature after initial drying is a significant advance for the industry. Although this can be achieved via UV/EB-curable aqueous inkjet compositions there are innate issues with photoinitiator migratables and hydrolysis of acrylate groups, as previously mentioned. Furthermore, UV/EB-curable aqueous inkjet compositions require both a thermal drying stage and a second UV-, or EB-curing process, whereas the inks of the current invention require only a thermal drying/curing stage. As mentioned previously, the invention could also have applicability in other printing and coating applications where the resolubility conferred through the use of hydroxyl-functional PUDs would be advantageous, including flexographic printing.

Another key finding of this invention is that the inclusion of amines with boiling points in excess of 130° C., but below 250° C. (and more preferably less than 200° C.), such as N-methylethanolamine, N-methyldiethanolamine, dimethylethanolamine and 2-amino-2-methyl-1-propanol help to further promote the resolubility of the inks of the invention whilst allowing cure to take place after prints of the inks have been dried at temperatures below 100° C. Furthermore, where the inks of the invention are used in the printing of food packaging and include the optional additional amine, then dimethylethanolamine and 2-amino-2-methyl-1-propanol are especially well suited, due to having specific migration limits of 18,000 and 5,000 ppb respectively, according to current regulations.

Self-crosslinking polymers. Self-crosslinking polymers contain a functionality which is self-reactive, and thus do not require the use of a separate co-reactant per se. A self-crosslinking polymer is usually in the form of an aqueous dispersion or emulsion and is typically the product of at least two monomers that react with one another. For example, such a polymer may contain both a carbonyl and an amine functional group. There are several mechanisms by which a polymer can be self-crosslinking. One mechanism is by the use of phase separated polymers, such as core-shell polymers. The shell polymer is hydrophilic, while the core polymer is hydrophobic. The hydrophilic shell maintains the dispersion, while the hydrophobic core provides the reactive sites for crosslinking. Alternatively or in addition, self-crosslinking polymer emulsions can comprise crosslinkable functional groups attached to the polymer backbone in addition to a crosslinker (i.e. a polyfunctional species that reacts with said crosslinkable functional groups). Typically, in self-crosslinking acrylic polymer chemistry, polymers containing ketone groups crosslink at room temperature when combined with bi- or polyfunctional compounds that are reactive towards carbonyl. One example of these reactive compounds is bishydrazides. Such self-crosslinking acrylic emulsions are provided as one pack products. The self-crosslinking reaction, depending upon the acrylic type, may also be initiated by the evaporation of water upon drying, a change of pH of the vehicle, or by curing at elevated temperatures where the cross-linking reaction occurs faster or the reactive groups are de-blocked. In a preferred embodiment, the self-crosslinking polymers used in the present invention undergo self-crosslinking at room temperature (e.g. 25° C.). In other words, self-crosslinking polymers and polymer emulsions are species that undergo crosslinking when initiated by one of the above-mentioned methods, but otherwise can be stored long-term in a stable state without undergoing significant crosslinking. Self-crosslinking polymers and polymer emulsions do not require mixing with a crosslinking agent in order to undergo crosslinking.

Self-Crosslinkable Styrene-Acrylics (SCSAs): These are preferably used in the form of a resin dispersion (sometimes referred to as emulsions or latexes), although aqueous solution polymers may also be used. The invention encompasses SCSAs that utilize a variety of crosslinking chemistries, such as for example any of the aforementioned including; carbonyl-amine, carbonyl-metal complex, carboxylic acid-metal complex, methylol self-condensation, organosilane self-condensation. As well as the monomers, such as diacetone acrylamide, acetoacetoxy ethyl methacrylate, (meth)acrylic acid used to introduce the desired reactive functionality into the polymer these polymers may comprise any blend of styrene, styrene-derivative, methacrylate and acrylate monomers. Although referred to as styrene-acrylic (or styrene-(meth)acrylic) resin dispersions, it should be understood that these materials may be essentially free of any styrene or styrene derivative or indeed composed largely of styrene and styrene derivatives. The term is used in the context of the invention to describe any resin dispersion (or solution polymer) produced by the free radical polymerization of any blend of ethylenically unsaturated monomers. It should be appreciated by those skilled in the art that it is not necessary to list all the monomers that may be used to produce such dispersions and that other monomers including vinyl, (meth)acrylamide and olefinic types may also form part of such resin solutions and dispersions. The resin dispersions are commonly prepared by emulsion polymerization, using any blend of anionic and non-ionic surfactants; surfactant free dispersions are also covered by the invention, as are protective colloid stabilized dispersions. Solution styrene and acrylic copolymers are prepared usually by solution or solvent free radical polymerization processes, with the polymer being brought into aqueous solution by the neutralization of the acid groups of the polymer. The invention also covers the use of core-shell styrene-(meth)acrylic resin dispersions where the resin particles are heterophasic in nature with two or more discrete polymer phases. An optional embodiment of the invention includes the use of SCSAs which have been neutralized using amines with boiling points in excess of 130° C., in line with an optional embodiment of the invention. There is no restriction on the amine and examples include, but are not limited to; N,N-dimethylethanolamine, N-methylethanolamine, N-methyldiethanolamine, 2-amino-2-methyl-1-propanol, ethanolamine, propanolamine, triethanolamine. In a further embodiment it is preferred that the boiling point of the amine should be less than 250° C., and more preferably less than 200° C.

OH-PUD=Hydroxy-functional polyurethane dispersion. Both anionically and non-ionically stabilized OH-PUDs may be used. It is preferable that the OH-PUDs have an average particle size of less than 200 nm and more preferably less than 100 nm. Furthermore, the OH-PUD may be synthesized from any blend of polyol precursors including, but not limited to; polyether diols, polyester diols, polyacrylic diols and polycarbonate diols or blends and hybrids thereof. It is preferred that the OH-PUD should have a hydroxyl value of 50 mgKOH, or greater, based on the dry polymer weight.

T=Glass transition temperature. The glass transition temperature can be determined using differential scanning calorimetry (DSC) according to the process defined in ASTM E1356-08. The sample was maintained under an atmosphere of dry nitrogen for the duration of the scan. A flow rate of ml/min and Al pans were used. Samples (5 mg) were heated at 20° C./min from 20° C. to 350° C. The value of a Twas determined as the extrapolated onset temperature of the glass transition observed on the DSC scans (heat flow (W/g) against temperature (° C.)), as described in ASTM E1356-08.

Boiling point=unless otherwise specified, all boiling points are measured under standard atmospheric pressure of 100 kPa.

(w/w)=mass of component as a percentage of the total mass of the composition.